The present invention relates to electrocardiogram (ECG) systems, and in particular to ECG systems which can provide synthesized signals corresponding to signals generated from electrodes which provide the actual ECG signals.
ECG systems are well known, and provide information about the physiological status of a patient""s heart to a physician. More specifically, so called 12 lead ECG systems exist which provide twelve waveforms, called leads, to the physician. To provide such a 12 lead ECG, ten electrodes are placed on the patient""s body, and the signals from these electrodes are processed to provide the twelve leads, all in a known manner. These ten electrodes include four electrodes which provide signals processed to generate six of what are known as limb leads, and six electrodes which provide signals processed to provide six of what are known as precordial or chest leads.
In some cases, signals from all ten electrodes, necessary for proper derivation of a 12 lead ECG, are not available. For example, sometimes, electrodes work loose from, or slip on, a patient""s body and produce a null signal, or produce signals which are otherwise degraded to the point of being unusable. Furthermore, the location on the patient""s body at which one or more of the electrodes should be placed may be unavailable due to injury or surgery. As another example, in non-critical monitoring or telemetry conditions, modules providing only five or six electrodes are provided, both for patient comfort and to minimize the signal processing required.
In other cases, it may be desirable to place an electrode at a location on the body different from the normally used locations. For example, it may be desired to monitor the right side of the heart, or further around the left side.
In any case, electrodes are sometimes placed in incorrect locations, or two electrodes are reversed. Also electrodes may provide anomalous signals. For example, if an electrode has worked loose from, or slipped on, a patient, the signal from that electrode is not accurate. Alternatively, if a patient moves (e.g. rolls on his side, or sits up), the signal from all the electrodes, even if accurate, may provide anomalous results after analysis due to the patient movement. It is desirable under all the conditions described above to still provide the signals needed to generate the 12 lead ECG.
It is known that the signals representing the respective lead signals contain mutually redundant information. It is also known that, should one electrode be missing or malfunctioning, an appropriate combination of signals from the other electrodes and/or the other leads, which are available and functional, can be used to generate a synthesized signal which closely approximates the lead signal derived from the missing or malfunctioning electrode. To apply this technique, at least some portion of a full 12 lead ECG is recorded, during an analysis phase. The recorded signals are then processed to generate a function, which may be applied to the lead signals which are available, to synthesize a lead signal which approximates the lead signal which is missing or distorted beyond use. During a synthesis phase, this function is then applied to the available ECG lead signals. Using this technique, a missing lead may be synthesized.
In U.S. Pat. No. 5,058,598, issued Oct. 22, 1991 to Nicklas et al., a system is disclosed for synthesizing a desired precordial lead from what is termed a set of base leads. First, in an analysis phase, a set of ECG lead signals, including at least the set of base leads (in Nicklas et al, the base leads are leads I, II, and V2), and the precordial lead signal (other than V2) which is desired to be synthesized, is processed to generate coefficients for a linear equation. Then, in a synthesis phase, signals representing only the base leads are received, and the values of those base leads are substituted into the linear equation to derive values which represent the desired synthesized precordial lead signal. Nicklas et al. also discloses partitioning the ECG complex into segments (e.g. QRS, ST, etc.), and processing each segment separately to generate respective sets of coefficients for a separate linear equation corresponding to each segment. In this case, during the synthesis phase, values for each segment from the base leads are substituted into the appropriate linear equation, to derive values which represent the desired synthesized precordial lead signal in that segment.
In U.S. Pat. No. 5,490,515, issued Feb. 13, 1996 to Mortara, a system is disclosed for synthesizing a single specified lead from a set of eight lead signals. In the analysis phase, the set of eight lead signals, derived from respective electrodes, is received by the system and a coefficient table, having entries representing coefficients of a set of linear equations, is generated. In the synthesis phase, the coefficient table is then used to synthesize a selected one of the eight lead signals, based on the values of the other seven lead signals. Mortara also discloses simultaneously synthesizing more than one missing lead.
In neither of these systems is any indication of the accuracy of the synthesized signal provided to the operator. In addition, in neither of these systems is any information provided to the operator to alert the operator to anomalous signals, nor to assist in preparation for the ECG or in interpreting the displayed 12 ECG lead waveforms.
It is desirable to determine the accuracy of the synthesized signal relative to other potential synthesized signals, and to provide that information to the operator. It is further desirable to provide information to the operator to alert the operator to anomalies, and to assist in the preparation for the ECG and the interpretation of the results. In some cases, such as telemetered ECGs, it is further desirable to monitor patients with a minimum number of electrodes, while producing a full 12 lead ECG and maintaining a desired level of accuracy.
In accordance with principles of the present invention, an electrocardiogram (ECG) system provides a set of ECG lead signals. The system includes a source of a subset of ECG lead signals. A synthesizer, coupled to the ECG lead signal source, generates a set of synthesized ECG lead signals from the subset of ECG lead signals. Data is also generated representing the accuracy of the set of synthesized ECG lead signals.